Acute pyelonephritis is a bacterial infection of the kidneys that affects up to 2 percent of pregnant women. In most cases, the infection first develops in the lower urinary tract. If it’s not diagnosed and treated properly, the infection may spread from the urethra and genital area to the bladder and then to one or both kidneys.

Pregnant women are more likely to develop pyelonephritis than women who aren’t pregnant. This is due to physiological changes during pregnancy that can interfere with the flow of urine.

Normally, the ureters drain urine from the kidney into the bladder and out of the body through the urethra. During pregnancy, the high concentration of the hormone progesterone can inhibit contraction of these drainage ducts. Also, as the uterus becomes enlarged during pregnancy, it can compress the ureters.

These changes can lead to problems with proper drainage of urine from the kidneys, causing the urine to remain stagnant. As a result, bacteria in the bladder may migrate to the kidneys rather than being flushed out of the system. This causes an infection. The bacteria Escherichia coli (E. coli) is the usual cause. Other bacteria, like Klebsiella pneumoniae, the Proteus species, and Staphylococcus, can also cause kidney infections.

Typically, the first symptoms of pyelonephritis are a high fever, chills, and pain on both sides of the lower back.

In some cases, this infection causes nausea and vomiting. Urinary symptoms are also common, including:

Proper treatment of pyelonephritis may prevent serious problems. If untreated, it can lead to a bacterial infection in the bloodstream called sepsis. This can then spread to other parts of the body and cause serious conditions requiring emergency treatment.

Untreated pyelonephritis can also result in acute respiratory distress as fluid accumulates in the lungs.

Pyelonephritis during pregnancy is a leading cause of preterm labor, which puts the baby at high risk for serious complications and even death.

A urine test can help your doctor determine whether your symptoms are the result of a kidney infection. The presence of white blood cells and bacteria in urine, which can be viewed under a microscope, are both signs of infection. Your doctor can make a definitive diagnosis by taking bacterial cultures of your urine.

As a general rule, if you develop pyelonephritis during pregnancy, you’ll be hospitalized for treatment. You’ll be given intravenous antibiotics, probably cephalosporin drugs such as cefazolin (Ancef) or ceftriaxone (Rocephin).

If your symptoms don’t improve, it may be that the bacteria causing the infection are resistant to the antibiotic you’re taking. If your doctor suspects that the antibiotic isn’t able to kill the bacteria, they may add a very strong antibiotic called gentamicin (Garamycin) to your treatment.

Blockage within the urinary tract is the other main cause of treatment failure. It’s typically caused by a kidney stone or physical compression of the ureter by the growing uterus during pregnancy. Urinary tract obstruction is best diagnosed through an X-ray or an ultrasound of your kidneys.

Once your condition begins to improve, you may be allowed to leave the hospital. You’ll be given oral antibiotics for 7 to 10 days. Your doctor will choose your medication based on its effectiveness, toxicity, and cost. Drugs such as trimethoprim-sulfamethoxazole (Septra, Bactrim) or nitrofurantoin (Macrobid) are often prescribed.

Recurrent infections later in pregnancy aren’t uncommon. The most cost-effective way to lower your risk of recurrence is to take a daily dose of an antibiotic, such as sulfisoxazole (Gantrisin) or nitrofurantoin monohydrate macrocrystals (Macrobid), as a preventive measure. Remember that drug dosages may vary. Your doctor will prescribe what’s right for you.

If you’re taking preventive medication, you should also have your urine screened for bacteria each time you see your doctor. As well, be sure to tell your doctor if any symptoms return. If the symptoms return or if a urine test shows the presence of bacteria or white blood cells, your doctor may recommend another urine culture to determine if treatment is necessary.